The industry of telephony was the first to provide protection against transient events such as voltage spikes and current surges as appear from time to time on transmission lines. Such transient events may be summarily described as unwanted and individually unpredictable burst occurrences of electrical or electromagnetic energy. The effect of any such transient event occurrence upon a station set, a transmission line, or a central facility may range from a bit of noise in a communications signal, to destruction of physical apparatus. In a central office telephone exchange environment, transmission facilities, such as telephone lines and trunks are each physically connected to the telephone exchange apparatus via an interface circuit. The primary requirement of the interface circuit is that of passing the desired communications signals between the transmission line and associated apparatus while shielding the apparatus from any potentially destructive voltages or currents. This primary requirement is somewhat complicated in many applications where the interface circuit must also provide either a supply or a termination of energizing current for operation and supervision of the transmission line.
The problems of meeting these requirements with efficient and commercially competitive means has continued to be a focus of research and design development, from the very beginning of the telephone industry. However, the advent of electrically fragile semiconductor based telecommunications transmission terminals and telephone exchange switching facilities in the past decade or so, has served to amplify the severity of these requirements. For example, a comparatively robust step by step central office, crossbar central office, or electromechanical key system, in days of yore, was typically interfaced with a telephone line by means of a rather bulky transformer. The bulk of the transformer was dictated by the requirement that the transformer's magnetic core be large enough to carry the magnetic flux associated with direct line current, without the core becoming saturated. Consequently there was abundant space for transformer windings and winding insolation sufficient to withstand spikes in excess of ten thousand volts. Although the windings would inductively couple intolerable amounts of transient energy by today's standards, the electromechanical telephone exchanges of the time were inherently robust and such was of little practical concern. This is in stark contrast to the interface requirements of a typical modern pulse code modulated time division multiplex (PCM TDM) telephone switching system. These interface requirements were satisfied by a new approach to the interface problem, as exemplified by V. V. Korsky in U.S. Pat. No. 4,103,112 issued on July 25, 1978, and titled "Telephone Line Circuit With Differential Loop Current Sensing And Compensation". Korsky's circuit employed precision direct loop current flux cancellation means to drastically reduce the core size required in the transformer. A miniature core limits the amount of transient energy that can be inductively coupled across the transformer windings. Any energy so coupled is of a relatively moderate magnitude, the bulk of which can be directed to ground by convenient and inexpensive means. For example, one such means is that of silicon diodes, of a two or three ampere peak current rating, arranged in a clamping circuit, and being connected between the transformer and onfollowing circuitry.
The size reduction of the transformer core and windings also necessitated a reduction of winding insulation thickness as compared to the bulky transformer. Consequently the smaller windings are able to withstand voltage spikes of only a few thousand volts before breakdown occurs. Hence, it is preferred and considered essential by some operating telephone companies, that any telephone facility or apparatus coupled to a telephone line or a trunk must be protected by some arrangement of devices intended for absorbing any extreme portion of the energy of a transient burst, before the extreme energy reaches any transformer windings.
In a central office environment, the devices of choice are usually gas tubes. Gas tubes, as stated in the "SURGE PROTECTION TEST HANDBOOK" published in 1982 by the KeyTek Instrument Corp. of Burlington, Mass., 01803, U.S.A., are inherently robust and reliable devices which represent a refinement of uncontrolled flashover that occurs between terminals in air. Gas tubes are available with closely predetermined breakdown characteristics from around one hundred to several thousand volts. The breakdown characteristics are determined by electrodes of predetermined design shape and spacing which reside in an envelope of one or more gases at a suitable pressure.
A less reliable, but least costly and most widely used protection device in the telephone industry, is the well known carbon block. Carbon blocks have proven to be an effective means of protecting personnel and property from energy surges on transmission lines. Although the carbon block has initially reliable operating characteristics, after repeated breakdowns it is predictably unpredictable, as far as the protection of fragile electronic equipment is concerned. In spite of this, it is still the device of choice for the protection of low cost apparatus such as subscriber telephone sets and the like.
Until recently the typical key telephone system has traditionally been little more than a collection of telephone sets arranged to share several telephone lines connected to a central office. In such arrangements, carbon block protection has satisfactorily fulfilled the requirements of most operating telephone companies. However now, subscriber owned computer controlled solid state key like telephone systems have become very popular. These newer key systems tend to be more vulnerable to suffering catastrophic damage as a result of a transient event occurrence, in spite of the standard use of carbon block protectors at the subscriber's premises.
Transient event occurrence vulnerability has been identified in the trunk interface circuit which transformer couples signals between tip and ring (CO) trunk leads and the solid state key like telephone system. In these newer key systems, the size of the interface transformer has been further reduced, to reduce the manufacturing cost. Lesser transformer core size dictated lesser winding insulation, such that typical breakdown occurs in the neighborhood of a thousand volts. Theoretically, this should be sufficient, on the assumption that the carbon block protection actually meets the accepted standard. In practice however, this is not sufficient in every instance to provide for adequate protection in every installation.
A contributing factor to inadequate protection of electronic equipment appears to be a wide variance of actual installation conditions and configurations of carbon block protection apparatus, in combination with various line and trunk entrances at different subscriber premises. An installation is usually put in place by a telephone craftsperson. The telephone craftsperson is faced with a wide variety of physical installation situations some of which are without an easily accessible grounding location. Furthermore, the telephone craftsperson may be unaware of the potentially deleterious effects of inductive reactance, as may accompany a long run of light gauge grounding wire in association with a carbon block protector installation. In some instances a long ground wire run, short of installing a ground rod, cannot be avoided. As may be appreciated, other than the transformer winding insulation in the interface circuit in such installations, there is little or nothing to impede a five or ten microsecond leading edge of transient impulse energy from damaging an electrically fragile telephone apparatus. Even if a potentially ineffective protection situation is identified, most operating telephone companies are reluctant to incur the cost of on site engineering of sophisticated nonstandard protection arrangements. Hence in the case of a new installation of a subscriber owned apparatus, the subscriber must incur such cost or risk almost certain significant damage to the new telephone system at sometime in the future.
It is an object of the invention to provide an arrangement whereby the vulnerability of telecommunications apparatus to transient electrical events in association with a transmission line is reduced.
It is a further object of the invention to provide a line interface circuit for coupling communications signals between a telephone line and a telephone apparatus whereby the vulnerability of the apparatus to transient electrical events in association with a transmission line s effectively reduced.
The invention provides an overvoltage device, for use in combination with an electrical circuit wherein the electrical circuit includes a sheet of electrically insulating material having first and second surfaces with at least one of the surfaces carrying conductors of the electrical circuit. The conductors are spaced apart one from the other by at least a predetermined amount. The electrical circuit is operable in response to energizing current from a power supply connected with an earth ground, for terminating a transmission path. The overvoltage device includes a wall traversing the sheet of insulating material between the first and second surfaces and defines an opening therebetween. A first conductor is carried by the first surface and includes a first terminating edge which surrounds the opening and is spaced apart therefrom. A second conductor is carried by the second surface, and the wall and protrudes therebeyond to a second terminating edge. The second terminating edge is spaced from the first terminating edge by less than the predetermined amount.
One example of the invention is the telephone system installation at a telephone subscriber premises, for providing communications by way of at least one transmission line having tip and ring leads extending to a service entrance at the telephone subscriber premises. The installation comprises a telephone system apparatus being installed within the telephone subscriber premises and including an interface circuit having a direct current isolation device of limited withstand characteristics, for coupling alternating current information signals with the tip and ring conductors of the transmission line. Overvoltage breakdown devices are connected between any convenient electrically conductive element associated with earth ground and the tip and ring conductors. A power supply means is connected to power leads and earth ground associated with a local electrical supply utility, for supplying energizing current for operation of the telephone system apparatus. A sheet of electrically insulating material including first and second surfaces and a pair of openings each being defined by a peripheral wall traversing the insulating material between the first and second surfaces carries the direct current isolation device, fixed against one of the surfaces. Tip and ring terminating edges portions are carried by the peripheral walls of the respective openings and by the first surface. A ground electrical conductor is carried by the first surface and includes ground terminating edges. One and another of the ground terminating edges surrounds the tip terminating edge portion and the ring terminating edge portion respectively and is spaced apart therefrom by a distance characterized by a breakdown voltage of less than said withstand voltage. Tip and ring electrical conductors each are carried by the second surface and are connected to the tip and ring terminating edge portions via respective ones of the pair of openings.
In an example, in accordance with the invention, a telephone system installation at a telephone subscriber premises is capable of providing communications by way of a transmission line having tip and ring leads extending to a service entrance at the telephone subscriber premises. The installation comprises a telephone system apparatus being installed within the telephone subscriber premises and including an interface circuit having a direct current isolation device of limited withstand characteristics for coupling alternating current information signals with the tip and ring conductors of the transmission line. Overvoltage breakdown devices are connected between any convenient electrically conductive element associated with earth ground and the tip and ring conductors. A power supply is connected to power leads and earth ground associated with a local electrical supply utility, for supplying energizing current for operation of the telephone system apparatus. The direct current insolation device and ground, and tip and ring electrical conductors, are fixed to the surface of a sheet of electrically insulating material for supporting same. The ground and tip and ring electrical conductors are each bounded by a peripheral edge, with each peripheral edge being separate from any other peripheral edge by a first predetermined amount. The ground conductor is connected to the earth ground, and the tip and ring conductors are connected between the direct current insolation device and the respective tip and ring leads of the transmission path. The ground electrical conductor includes first terminating edge portions and the tip and ring electrical conductors each include a second terminating edge portion. Each of the second terminating edge portions is arranged in juxtaposition with the surface of the sheet and is spaced apart from one of the first terminating edge portions by a second predetermined amount being less than said first predetermined amount.